It is known to monitor the physical characteristics of structures and bodies using sensors. One such application is the monitoring of oil wells to extract such information as temperature, pressure, fluid flow, seismic and other physical characteristics. The monitoring of oil wells presents certain challenges for conventional sensors because of the harsh environment in terms of high pressures and temperatures. Historically the monitoring of oil wells has been dominated by the use of electronic sensors with optical sensors being used to a lesser degree.
The presently used electronic sensors are limited for several reasons. First, the on-board electronics of such sensors must operate in a very hostile environment, which includes high temperature, high vibration and high external hydrostatic pressure. Second, electronics' inherent complexity renders them prone to many different modes of failure. Because early failure of the sensors results in time-consuming and expensive well intervention, such failures have traditionally caused a less than acceptable level of reliability when electronic sensors are used to monitor oil wells.
There are numerous other problems associated with the transmission of electrical signals within well bores. It is extremely difficult to seal the required insulated cables against exposure to well bore fluids, which are at high temperatures, high pressures, and are very corrosive. Electrical conductors damaged by the fluids that penetrate the insulating materials around the electrical conductors will typically short-circuit the electrical signals. Additionally, electrical transmissions are subject to electromagnetic interference in many production operations.
Accelerometers are used to measure down-hole seismic disturbances to determine the acoustic wave characteristics of underground layers in proximity of the well bore. An accelerometer may be considered as a mass-spring transducer housed in a sensor case with the sensor case coupled to a moving body, the motion of which is inferred from relative motion between the mass and the sensor case. Such accelerometers may be analyzed by considering the relative displacement of the mass as being directly related to the acceleration of the case and therefore to the acceleration of the earth in proximity to the well bore. An array of accelerometers may be placed along the length of a well bore to determine a time-dependent seismic profile.
One prior art accelerometer is a piezoelectric-based electronic accelerometer. Piezoelectric accelerometers typically suffer from the above-identified problems common to electronic sensors. Additionally, most high performance piezoelectric accelerometers require power at the sensor head. Also, multiplexing of a large number of sensors is cumbersome and tends to incur significant increases in weight and volume with a decrease in reliability.
It is also known to use optical interferometers for the measurement of acceleration of certain structures. It is also well known that fiber optic interferometric accelerometers can be designed with high responsiveness and reasonably low detection thresholds. Some prior art fiber optic accelerometers include interferometric fiber optic accelerometers based on linear and nonlinear transduction mechanisms, circular flexible disks, rubber mandrels and liquid-filled-mandrels. Some of these fiber optic accelerometers have displayed very high acceleration sensitivity (up to 104 radians/g), but tend to utilize a sensor design that is impractical for many applications. For example, sensors with a very high sensitivity typically have a seismic mass greater than 500 grams, which seriously limits the frequency range in which the device may be operated. Additionally, these devices are so bulky that their weight and size renders them useless in many applications. Other fiber optic accelerometers suffer from high cross-axis sensitivity, low resonant frequency. Many fiber optic accelerometers require an ac dither signal or tend to be bulky (>10 kg), expensive and require extensive wiring and electronics. Even optical interferometers designed of special material or construction are subject to inaccuracies because of the harsh borehole environment and because of the very tight tolerances in such precision equipment.
For many applications, it is desirable that the fiber optic sensor is expected to have a flat frequency response up to several kHz (i.e., the device must have high resonant frequency). It is also desirable that the devices have high sensitivity, immunity from extraneous parameters (e.g., dynamic pressure). Finally, it is also desirable that the devices have a small foot print and packaged volume that is easily configured in an array (i.e., easy multiplexing).